Reaction Enthalpy Calculator

Work out reaction enthalpy instantly with clear inputs, formula shown and shareable results.

About Reaction Enthalpy Calculator

Standard reaction enthalpy is the total formation enthalpy of the products minus that of the reactants, each weighted by its coefficient. A negative result means the products sit lower in energy and heat leaves the system. Scaling by the moles actually run converts the per-mole figure into the energy a given batch releases or requires.

Formulas

Hess summation

dH(rxn) = sum dHf(products) - sum dHf(reactants)

Batch energy

energy = dH(rxn) x moles of reaction

Worked example

With the default values below, Reaction Enthalpy Calculator returns Reaction Enthalpy of -78.10. Change any input above and every figure here is recalculated in your browser.

Inputs used

Sum of product formation enthalpies: -1,206.9 kJ/mol
Sum of reactant formation enthalpies: -1,128.8 kJ/mol
Moles of reaction runminimum 0 mol: 2 mol

Results

Reaction Enthalpy: -78.10 kJ/mol
Energy For The Stated Moles: -156.20 kJ
Thermal Character: Exothermic - heat is released
Same Value In kcal/mol: -18.666

Step by step

1Values used
2Hess summation
3Batch energy
4Reaction Enthalpy= -78.10 kJ/mol
5Energy For The Stated Moles= -156.20 kJ
6Thermal Character= Exothermic - heat is released
7Same Value In kcal/mol= -18.666

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Do elements need a formation enthalpy?

No. An element in its standard state has dHf = 0 by definition, so it contributes nothing to either sum.

Why is a negative value exothermic?

Enthalpy is measured from the system's point of view. Losing enthalpy to the surroundings makes the change negative and the flask warm.